Liquid crystal display devices, which achieve light weight, thin profile, and low power consumption, have been widely used as essential articles in daily life and business, such as mobile applications (e.g. smartphones, tablet terminals), various monitors, and large TV screens. Such liquid crystal display devices have been developed for further improving the display quality to achieve larger viewing angles and improved contrast, and for providing more functions.
Current liquid crystal display devices display images as follows. First, the alignment of liquid crystal molecules is controlled by applying an electric field to liquid crystal. The polarization of the light passing through the liquid crystal layer is thereby changed to adjust the amount of light passing through the polarizer.
The display performance of a liquid crystal display device is affected by the alignment state of liquid crystal molecules in an electric field applied and the size and direction of the electric field. Liquid crystal display devices can employ various display modes depending on the alignment state of liquid crystal molecules under no electric field and the direction of the electric field to be applied.
Examples of the display mode of a liquid crystal display device include a vertical alignment (VA) mode in which liquid crystal molecules having negative anisotropy of dielectric constant are aligned vertically to the substrate surface; and an in-plane switching (IPS) mode and a fringe field switching (FFS) mode in each of which liquid crystal molecules having positive or negative anisotropy of dielectric constant are aligned horizontally to the substrate surface and a transverse electric field is applied to the liquid crystal layer.
In particular, in multi-domain vertical alignment (MVA) mode which use liquid crystal molecules having negative anisotropy of dielectric constant and are provided with banks (ribs) and electrode-free parts (slits) as components for alignment control, the liquid crystal can be aligned in multiple directions under an electric field without a rubbing treatment to the alignment film. The mode thus achieve excellent viewing angle characteristics. In conventional MVA liquid crystal display devices, unfortunately, the boundaries of the divisions for the alignment of liquid crystal molecules may overlap with the upper parts of protrusions or slits, whereby the display devices may have reduced transmittance in a white display mode and may display dark lines on the screen. Thus, conventional MVA liquid crystal display devices still have room for improvement.
Alignment-stabilizing techniques using a polymer (hereinafter, also referred to as polymer sustained (PS) techniques) have been proposed as methods for manufacturing liquid crystal display devices with a high luminance and a high response speed (e.g. Patent Literature 1 to Patent Literature 8). Among these techniques, in a pre-tilt-angle-imparting technique using a polymer (hereinafter, also referred to as the polymer sustained alignment (PSA) technique), a liquid crystal composition containing a polymerizable component such as a polymerizable monomer or oligomer is sealed between substrates. The polymerizable component including the monomer is polymerized while the liquid crystal molecules are tilted (inclined) by a voltage applied across the substrates, thereby forming a polymer. This process allows the liquid crystal molecules to have a certain pre-tilt angle even after finishing voltage application, and can align the liquid crystal molecules in a certain direction.
Examples of other liquid crystal display elements including a polymerizable monomer include polymer-stabilized blue phase liquid crystal display elements (e.g. Non Patent Literature 1, Patent Literature 9).
Another example is a disclosure of a liquid crystal display element, in which a polymerizable monomer itself is proposed. The liquid crystal display element includes a pair of substrates to which voltage is applicable and which constitute a cell with a controlled gap, and a liquid crystal layer which is sandwiched between the pair of substrates and is to be driven by voltage application. The liquid crystal layer includes a liquid crystal composition (A) and a cured product. The liquid crystal composition (A) contains one or more kinds of liquid crystal molecules. The cured product is obtained by polymerizing one or more kinds of polymerizable compounds with energy line, heat, or a combination of these, and controls the inclination of the liquid crystal molecules. At least one of the polymerizable compounds contains two or more polymerizable functional groups in a molecule, and the two or more polymerizable functional groups are polymerizable compounds (A) which are at least two different kinds of functional groups (e.g. Patent Literature 10).